Contact fusing of thermoplastic powder xerographic images is typically carried out by the use of a heated roll that conducts heat through its outer surface or sheath to a thermoplastic image held in contact therewith by a back-up pressure roll. The heated image particles soften, flow, and are thereby fixed onto the sheet upon which they are supported. Insufficient heating leaves the toner particles in a partially powder form so that they can separate with part of the ink particles going to the heated roll and part of the particles remaining on the support sheet. On the other hand, excessive heating of the toner produces a liquid having low cohesive strength allowing separation with the result again being the transfer of some toner to the heated fuser roll. In both cases the ultimate result is that some of the toner deposited on the fuser roll may be re-deposited on the subsequent image sheet creating unsightly smudged images. The tendency for offset may be reduced by minimizing the adhesive nature of the heated fuser roll.
Employing fuser rolls having surfaces of a material such as polytetrafluoroethylene, which is known to have low surface energy, either alone or in combination with lubricant liquids such as silicone oil, provides such a non-adhesive surface. While such materials do provide a relatively non-adhesive surface they create certain limitations on the overall performance of the contact fuser. For example, the operative or wear life of a fuser roll having such a pre-applied permanent offset preventing release surface layer will be limited by the life of that layer necessitating frequent servicing. Furthermore, imperfections created in the surface layer by constant use diminish its release life and limit the speed of operation.
This invention provides an adhesion preventing layer that can be continuously applied to the bare metal surface of the fuser roll thereby increasing its wear life to that of the metallic surface and also considerably increasing the release life. Additionally, since a new non-adhesive layer is being deposited during each xerographic reproduction process, increased speeds of operation can be achieved.